The present invention relates generally to displays, and more particularly, using pulse-width modulation to drive one or more display elements of an electro-optical display.
Pulse-width modulation (PWM) has been employed to drive liquid crystal (LC) displays. A pulse-width modulation scheme may control displays, including emissive and non-emissive displays, which may generally comprise multiple display elements. In order to control such displays, the current, voltage or any other physical parameter driving the display element may be manipulated. When appropriately driven, these display elements, such as pixels, normally develop light that can be perceived by viewers.
In an emissive display example, to drive a display (e.g., a display matrix having a set of pixels), electrical current is typically passed through selected pixels by applying a voltage to the corresponding rows and columns from drivers coupled to each row and column in some display architectures. An external controller circuit typically provides the necessary input power and data signal. The data signal is generally supplied to the column lines and is synchronized is to the scanning of the row lines. When a particular row is selected, the column lines determine which pixels are lit. An output in the form of an image is thus displayed on the display by successively scanning through all the rows in a frame.
For instance, a spatial light modulator (SLM) uses an electric field to modulate the orientation of an LC material. By the selective modulation of the LC material, an electronic display may be produced. The orientation of the LC material affects the intensity of light going through the LC material. Therefore, by sandwiching the LC material between an electrode and a transparent top plate, the optical properties of the LC material may be modulated. In operation, by changing the voltage applied across the electrode and the transparent top plate, the LC material may produce different levels of intensity on the optical output, altering an image produced on a screen.
Typically, a SLM, such as a liquid crystal on silicon (LCOS) SLM, is a display device where a LC material is driven by circuitry located at each pixel. For example, when the LC material is driven, an analog pixel might represent the color value of the pixel with a voltage that is stored on a capacitor under the pixel. This voltage can then directly drive the LC material to produce different levels of intensity on the optical output. Digital pixel architectures store the value under the pixel in a digital fashion, e.g., via a memory device. In this case, it is not possible to directly drive the LC material with the digital information, i.e., there needs to be some conversion to an analog form that the LC material can use.
A SLM such a LCOS SLM operates by applying a bias across the LC material to change the optical properties of the material. Due to the nature of the LC material, it must always be driven in a direct current (DC) balanced fashion; that is, the net bias across the material integrated over time must be zero.
Assuming the time over which the device modulates is constant, maintaining DC balance is straight forward. However, in real-world systems, this is often not the case. There are two primary reasons that DC balance is difficult to maintain. First, changes in the length of a video frame due to clock domain crossings as the data travels from the video source to the display can occur. Second, changes in the length of a video frame due to synchronization with a color management system for the display, such as a color wheel, can also occur.
In each of these cases, the display is confronted with a situation where there may be a slight jitter in a desired amount of time for which to modulate a give frame. In digital microdisplays such as a LC display operating with a pulse-width modulated (PWM) waveform, the issue is further complicated in that digital displays quantize time and thus may only change the modulation duration in discrete intervals. A need thus exists to better drive a display with digital modulation signals.